Automatic color photographic printers, such as the CLAS35 printer made by Eastman Kodak Company are known to employ color balance algorithms for determining the amounts of exposure light in each of a plurality of primary colors for use in exposing film images onto copy paper. In the CLAS35 printer, the algorithms rely on film channels with specific parameter values that are uniquely associated with each of the different film types encountered in the population of orders processed by the printer. This requires that a large library of parameter values be maintained and that the parameter values be updated as new film types are introduced. Additionally, changes in the photometric properties of existing film types caused, for example, by film processing errors or film storage at high temperatures or for long periods of time can introduce printing errors that are not compensated for by the film type parameter values.
U.S. Pat. No. 4,279,502 discloses a method of determining color balanced copying light amounts from photometric data derived directly from the film without the use of film type specific parameter values. In this method, first and second color density difference functional correlations are established from density values denoting the results of measurements at a plurality of regions of the film strip which includes the original image being copied. These correlations are then used for determining the copying light amounts for most of the originals on the film strip. The light amounts for originals containing illuminant error or color dominant subjects are selected differently using empirically determined threshold values. To be effective, this method requires the establishment of two different, independent functional relationships which may not capture the correct correlation among three primary color densities in the original image. There is therefore a need for a method of determining color balanced amounts of copying light that is based on the establishment of a single functional relationship among the image colors that captures the correlation among the three primary color densities.